In recent years, cellulose materials attract great attention as biomass materials which can be produced in the largest quantity on earth, and materials which are biodegradable in the environment. As cellulose fibers, short fibers of cotton and hemp, which are produced in nature, have been used for spinning from old times. General methods for obtaining a filament material, but not short fibers, include a method comprising dissolving cellulose such as rayon, lyocell, or the like in a special solvent system, and then spinning by a wet spinning process, and a method comprising dissolving a cellulose derivative such as cellulose acetate in an organic solvent such as methylene chloride or acetone, and then spinning by a dry spinning process while evaporating the solvent.
However, fibers obtained by the wet spinning process or dry spinning process have the problem of low productivity due to a low spinning rate, and are not necessarily friendly with the environment because organic agents used for producing fibers, such as carbon disulfide, acetone, methylene chloride, and the like, are highly likely to adversely affect the environment. Therefore, a melt spinning process not using an organic agent is necessary for obtaining low-environmental-load fibers using cellulose as a raw material.
Known examples of a thermoplastic cellulose composition capable of melt spinning and fibers comprising the composition include a thermoplastic cellulose composition containing cellulose acetate and a large amount of a water-soluble low-molecular-weight plasticizer such as glycerin or polyethylene glycol added for obtaining hollow yarns having permselectivity, and fibers comprising the composition, as disclosed in Japanese Unexamined Patent Application Publication Nos. 50-46921, 54-42420 and 62-250215. However, as disclosed in , for example, Japanese Unexamined Patent Application Publication No. 62-250215, the content of the low-molecular-weight plasticizer in the composition is as high as 50 to 59% by weight, and thus a heating loss at a spinning temperature is extremely large, thereby deteriorating spinability due to significant evaporation of the plasticizer. Therefore, plasticization of cellulose acetate by adding an external plasticizer has the problem of necessitating adding a large amount of external plasticizer for imparting sufficient plasticity to a composition. Furthermore, these methods are aimed at obtaining hollow yarns having selectivity, and thus the resultant fibers are very thick and have an external diameter of 200 to 300 μm. Therefore, these methods are not techniques for producing fibers having thermal fluidity and spinability enough for use as multi-filaments for clothing materials.
Also, known cellulose derivatives used as thermoplastic materials for extrusion molding and injection molding include so-called “acetate plastic compositions” each comprising cellulose acetate plasticized by adding a large amount of low-molecular-weight plasticizer such as dimethyl phthalate or triacetin. These compositions contain 30 to 50% by weight of plasticizer, and have poor melting properties and the problem of producing bleedout of the plasticizer added, and thus the composition cannot be used for melting spinning.
Furthermore, Japanese Unexamined Patent Application Publication Nos. 9-78339, 9-291414, 10-317228 and 11-506175 disclose cellulose acetate compositions plasticized by adding a plasticizer such as a ε-caprolactone derivative, and fibers obtained by melt-spinning the compositions. However, the compositions disclosed in these publications are obtained by a technique for plasticizing cellulose acetate by adding an external plasticizer thereto, and are required to contain a large amount of low-molecular-weight external plasticizer, thereby causing the problem of significantly evaporating the external plasticizer during melt spinning. When a composition contains a large amount of external plasticizer, the external plasticizer is likely to bleed out in a heating step for thermal setting or dyeing and finishing, or in use as a final product.
On the other hand, as an internal plasticization method of effecting direct graft reaction with a cellulose derivative without adding an external plasticizer, a method of producing a polymer by ring-opening graft polymerization of ε-caprolactone as a main material with a cellulose acetate main chain is known (Japanese Unexamined Patent Application Publication Nos. 58-225101, 59-86621, 7-179662, 11-255801, etc.). Although this polymer has no probability of bleedout of a plasticizer, a side chain is mainly composed of polycaprolactone, thereby causing fluidization of the side chain at a temperature of as low as about 60° C. Therefore, the polymer cannot satisfy minimum heat resistance required for fibers.
On the other hand, Japanese Unexamined Patent Application Publication No. 6-287279 discloses a method of producing a cellulose derivative by using, as a graft monomer, a lactide which is a dimer of lactic acid. Cellulose acetate grafted with lactide has no feel of sliminess, but has the problem of deteriorating heat resistance deteriorates, and increasing brittleness when an amount of polylactic acid graft side chains is excessively large. Examples of graft polymers disclosed in the specification of Japanese Unexamined Patent Application Publication No. 6-287279 are obtained by using 900 to 4900 wt % of L-lactide for cellulose acetate, and the resultant polymer compositions have extremely low melt viscosity at 200° C. These polymer compositions have the problem of deteriorating spinning performance because of an excessively low nozzle back pressure in melt spinning of the compositions.
Japanese Unexamined Patent Application Publication No. 11-240942 discloses a mixed composition comprising a cellulose ester or a cellulose ether grafted with lactide, and a plasticizer. However, this composition contains the low molecular-weight plasticizer, and thus has the problem of bleedout of the plasticizer during use of products.
With respect to a method of producing fibers by melt spinning, Japanese Unexamined Patent Application Publication Nos. 9-78339, 9-291414, 10-317228 and 11-506175 disclose techniques of melt-spinning cellulose acetate plasticized with a ε caprolactone derivative, or the like. However, these methods are production methods using high-speed air, and thus easily cause variations in the take-up speed with variations in air pressure, thereby increasing size irregularity of the obtained fibers. Therefore, the methods are not yet satisfactory spinning methods in view of application to clothing fibers. Furthermore, the methods are methods of “stretching and filamentation by using high-speed air, and then winding, or filamentation and then volume collection on a collecting support surface to form a web”. In these methods, convergence is not performed in a spinning step because filamentation is performed after spinning. Therefore, when the spinning rate is high, or a single yarn has a small size, spinning tension applied to fibers is liable to increase, thereby easily causing the problem of causing troubles such as yarn breakage and single yarn flow in a process. Furthermore, when a yarn is supplied from a package of a wound yarn to a higher processing apparatus such as a drawing machine, a weaving machine, a knitting machine, or the like, there is the problem of insufficient releasability. Also, since the winding method uses high-speed air, the winding speed easily varies with variations in the air pressure, thereby increasing size irregularity of the obtained fibers. Therefore, the fibers are not said to be suitable for clothing fibers.
Therefore, a cellulose derivative composition capable of melt spinning with high efficiency and causing no evaporation of a plasticizer has not yet been known, and there has been no method of providing cellulose derivative composition fibers having good mechanical properties and uniformity by melt spinning.